U.S. patent application number 15/982137 was filed with the patent office on 2018-11-22 for battery balancing and current control.
The applicant listed for this patent is Galley Power LLC. Invention is credited to Bradley M. Lehman, Peng Li, Liang Yan.
Application Number | 20180337536 15/982137 |
Document ID | / |
Family ID | 64272681 |
Filed Date | 2018-11-22 |
United States Patent
Application |
20180337536 |
Kind Code |
A1 |
Li; Peng ; et al. |
November 22, 2018 |
Battery Balancing and Current Control
Abstract
A circuit provides for regulating charge and discharge current
of a battery. A bypass circuit is connected to a terminal of the
battery and connected in parallel with a load switch. The bypass
circuit may selectively direct a bypass current around the load
switch. A controller can operate in plural modes to control the
bypass circuit. In a first mode, the controller controls one or
more parameters of the bypass current based on values corresponding
to a current at the terminal, a voltage at the terminal, and/or a
temperature of the battery. In a second mode, the controller
controls the bypass circuit to disable the bypass current.
Inventors: |
Li; Peng; (Bolton, MA)
; Yan; Liang; (Fremont, CA) ; Lehman; Bradley
M.; (Belmont, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Galley Power LLC |
MARLBOROUGH |
MA |
US |
|
|
Family ID: |
64272681 |
Appl. No.: |
15/982137 |
Filed: |
May 17, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62507764 |
May 17, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02J 7/0047 20130101;
H01M 10/425 20130101; H01M 10/63 20150401; H01M 10/443 20130101;
H02J 7/007192 20200101; H02J 7/0029 20130101; Y02E 60/10 20130101;
H02J 7/0048 20200101; H02J 7/00718 20200101; H02J 7/0091 20130101;
H02J 7/00 20130101; H02J 7/00304 20200101; H01M 2010/4271 20130101;
H01M 10/486 20130101; H02J 7/0016 20130101; H02J 2207/20 20200101;
H02J 7/007194 20200101 |
International
Class: |
H02J 7/00 20060101
H02J007/00 |
Claims
1. A circuit comprising: a bypass circuit connected to a terminal
of a battery and connected in parallel with a load switch, the
bypass circuit configured to selectively direct a bypass current
around the load switch; and a controller configured to: 1) in a
first mode, control at least one parameter of the bypass current
based on values corresponding to at least one of a current at the
terminal, a voltage at the terminal, and a temperature of the
battery, and 2) in a second mode, control the bypass circuit to
disable the bypass current.
2. The circuit of claim 1, wherein the bypass current is less than
a current passed by the load switch in an on state.
3. The circuit of claim 1, wherein the load switch is off in the
first mode, and wherein the load switch is on in the second
mode.
4. The circuit of claim 1, wherein the bypass circuit includes a
bidirectional converter configured to pass the bypass current.
5. The circuit of claim 4, wherein the bidirectional converter
includes at least one of a buck-boost converter, a boost-buck
converter, a buck converter, a boost converter, and a dual-active
bridge (DAB) converter.
6. The circuit of claim 1, wherein the controller, in the first
mode, controls the bypass current based on the temperature.
7. The circuit of claim 1, wherein the controller, in the first
mode, controls the bypass current based on a difference between the
temperature and a temperature of at least one additional
battery.
8. The circuit of claim 1, wherein the controller, in the first
mode, controls the bypass circuit to convert a discharge current to
an output current having a higher voltage and a lower current.
9. The circuit of claim 1, wherein the controller, in the first
mode and during charging of the battery, lowers the bypass current
in response to the temperature of the battery passing a
threshold.
10. The circuit of claim 1, wherein the controller switches between
the first and second modes based on a state of the load switch.
11. The circuit of claim 1, wherein the controller controls the
bypass current based on a received command signal indicating an
operational profile, the operational profile indicating at least
one of a discharge current, discharge voltage, charging current,
and a threshold battery temperature.
12. The circuit of claim 1, wherein the controller controls the
bypass current based on a status signal, the status signal
indicating at least one of temperature, current, capacity,
impedance and voltage at another battery.
13. The circuit of claim 1, wherein the controller is further
configured to inject a perturbation signal into the bypass current,
the perturbation signal altering the bypass current in a manner
indicating impedance of the battery.
14. The circuit of claim 1, wherein the controller is further
configured to calculate impedance of the battery based on a change
in at least one of voltage and current at the terminal.
15. The circuit of claim 1, wherein the at least one parameter of
the bypass current includes at least one of a magnitude and a
frequency of the bypass current.
16. The circuit of claim 1, wherein the bypass circuit is connected
to the terminal of the battery via at least one intermediary
circuit elements.
17. The circuit of claim 1, wherein, in the second mode, the bypass
circuit directs a leakage current around the load switch, the
leakage current having a magnitude less than a magnitude of the
bypass current.
18. The circuit of claim 1, wherein the controller is further
configured to receive at least one of a detected current at the
terminal, a detected voltage at the terminal, and a detected
temperature of the battery.
19. A battery system comprising: a plurality of cells coupled to a
first terminal and a second terminal; a load switch coupled to the
first terminal; a bypass circuit coupled to the first terminal and
in parallel with the load switch, the bypass circuit configured to
direct a bypass current around the load switch; and a controller
configured to selectively enable the bypass current.
20. The system of claim 19, further comprising a plurality of cell
balancing circuits, each of the plurality of cell balancing
circuits being coupled to a respective one of the plurality of
cells and configured to selectively discharge the respective cell
based on a command from the controller.
21. The battery system of claim 19, wherein the controller is
further configured to: 1) in a first mode, control the bypass
current based on at least one of a discharge current at the
terminal, a charge current at the terminal, a voltage at the
terminal, and a temperature of the battery, and 2) in a second
mode, control the bypass circuit to disable the bypass current.
22. A battery management system comprising: a first bypass circuit
configured to be coupled to a terminal of a first battery and in
parallel with a first load switch, the first bypass circuit
configured to selectively direct a first bypass current around the
first load switch; a second bypass circuit configured to be coupled
to a terminal of a second battery and in parallel with a second
load switch, the second bypass circuit configured to selectively
direct a second bypass current around the second load switch; and a
controller configured to selectively enable the first and second
bypass currents based on a status of at least one of the first and
second batteries.
23. The system of claim 22, wherein the status includes at least
one of a discharge current, a charge current, an output impedance,
and a detected temperature.
24. The system of claim 22, wherein, during a discharge of the
first and second batteries, the controller is configured to control
the first bypass current based on a difference between detected
output impedances of the first and second batteries.
25. The system of claim 22, wherein, during a discharge of the
first and second batteries, the controller is configured to control
the first bypass current based on a difference between detected
temperatures of the first and second batteries.
26. The system of claim 22, wherein the controller selectively
enables the first bypass current in response to an indication of a
difference in capacity between the first and second batteries.
27. The system of claim 22, wherein the controller selectively
enables the first bypass current based on information about at
least one of the first and second batteries.
28. The system of claim 27, wherein the information includes at
least one of a model number, cell chemical composition, a date of
manufacture, and a number of charge and discharge cycles.
29. The system of claim 22, wherein the controller is further
configured to inject respective perturbation signals into the
bypass current, the perturbation signals altering the first and
second bypass currents in a manner indicating impedance of the
first and second batteries.
30. The system of claim 29, wherein the controller controls the
respective perturbation signals to cancel out one another at a
combined battery output.
31. The system of claim 22, wherein the first and second bypass
circuits are communicatively coupled to the controller via a
controller area network (CAN) bus.
Description
RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 62/507,764, filed on May 17, 2017. The entire
teachings of the above application are incorporated herein by
reference.
BACKGROUND
[0002] Batteries are widely used to power various loads, and a
range of battery chemistries can be implemented in applications
depending on considerations such as safety, performance, stability,
or cost. Configuring multiple batteries to a common load is a way
to increase power capability. However, when batteries with
different properties are coupled to a common load, the
charging/discharging stress may not be distributed evenly, and
optimal battery performance may not be achieved. Circulating
current flowing among batteries at different SOCs or voltages can
occur, and can result in over-charging, over-discharging, early
failure of the batteries.
[0003] Typical batteries include a battery management system (BMS)
to control balancing of multiple battery cells. The BMS controls
charging and discharging of the battery cells in order to maintain
the battery cells at a common voltage or state of charge.
SUMMARY
[0004] Example embodiments include a circuit for regulating charge
and discharge current of a battery. The circuit may include a
bypass circuit and a controller. The bypass circuit may be
connected to a terminal of the battery and connected in parallel
with a load switch. The bypass circuit may be configured to
selectively direct a bypass current around the load switch. The
controller may be configured to operate in plural modes to control
the bypass circuit. In a first mode, the controller may control one
or more parameters of the bypass current based on values
corresponding to at least one of a current at the terminal, a
voltage at the terminal, and a corresponding temperature of the
battery. In a second mode, the controller may control the bypass
circuit to disable the bypass current.
[0005] In further embodiments, the bypass current may be less than
a current passed by the load switch in an on state. The load switch
may be off in the first mode, and may be on in the second mode. The
bypass circuit may include a bidirectional converter configured to
pass the bypass current. The bidirectional converter includes at
least one of a buck-boost converter, a boost-buck converter, a buck
converter, a boost converter, and a dual-active bridge (DAB)
converter.
[0006] In the first mode, the controller may control the bypass
current based on the corresponding temperature or a difference
between the corresponding temperature and a temperature of at least
one additional battery. The controller, in the first mode, may
control the bypass circuit to convert the discharge current to an
output current having a higher voltage and a lower current. During
charging of the battery in the first mode, the controller may lower
the bypass current in response to the corresponding temperature of
the battery passing a threshold. The controller may switch between
the first and second modes based on a state of the load switch.
[0007] The controller may control the bypass current based on a
received command signal indicating an operational profile, where
the operational profile may indicate a discharge current, discharge
voltage, charging current, and a threshold battery temperature. The
controller may also control the bypass current based on a status
signal, where the status signal indicates temperature, current,
capacity, impedance and/or voltage of another battery.
[0008] The controller may be further configured to inject a
perturbation signal into the bypass current, where the perturbation
signal alters the bypass current in a manner indicating impedance
of the battery. The controller may also calculate impedance of the
battery based on a change in at least one of voltage and current at
the terminal.
[0009] The parameters of the bypass current controlled by the
controller may include a magnitude and/or a frequency of the bypass
current. The bypass circuit may be connected to the terminal of the
battery via at least one intermediary circuit elements. In the
second mode, the bypass circuit may direct a leakage current around
the load switch, the leakage current having a magnitude less than a
magnitude of the bypass current. The controller may be further
configured to receive at least one of a detected current at the
terminal, a detected voltage at the terminal, and a detected
temperature of the battery.
[0010] Further embodiments include a battery system. A plurality of
cells may be coupled to a first terminal and a second terminal. A
load switch may be coupled to the first terminal. A bypass circuit
may be coupled to the first terminal and in parallel with the load
switch, where the bypass circuit may be configured to direct a
bypass current around the load switch. A controller is configured
to selectively enable the bypass current. The system may also
include a plurality of cell balancing circuits, where each of the
plurality of cell balancing circuits may be coupled to a respective
one of the plurality of cells and configured to selectively
discharge the respective cell based on a command from the
controller. The controller may be further configured to operate in
plural modes. In a first mode, the controller may control the
bypass current based on at least one of a discharge current
detected at the terminal, a charge current at the terminal, a
voltage at the terminal, and a corresponding temperature of the
battery. In a second mode, the controller may control the bypass
circuit to disable the bypass current.
[0011] Further embodiments may include a battery management system.
A first bypass circuit may be configured to be coupled to a
terminal of a first battery and in parallel with a first load
switch, where the first bypass circuit may be configured to
selectively direct a first bypass current around the first load
switch. A second bypass circuit may be configured to be coupled to
a terminal of a second battery and in parallel with a second load
switch, where the second bypass circuit may be configured to
selectively direct a second bypass current around the second load
switch. A controller may be configured to selectively enable the
first and second bypass currents based on a status of at least one
of the first and second batteries. The battery system can include
more than two batteries with same bypass circuit configurations as
described here.
[0012] In further embodiments of a battery management system, the
status may include a discharge current, a charge current, a
voltage, an output impedance, and a corresponding temperature.
During a discharge of the first and second batteries, the
controller may be configured to control the first bypass current
based on a difference between detected output impedances of the
first and second batteries. During the discharge, the controller
may be configured to control the first bypass current based on a
difference between corresponding temperatures of the first and
second batteries.
[0013] The controller may selectively enable the first bypass
current in response to an indication of a difference in capacity
between the first and second batteries. The controller may
selectively enable the first bypass current based on information
about at least one of the first and second batteries. The
information may include a model number, cell chemical composition,
a date of manufacture, and/or a number of charge and discharge
cycles. The controller may be further configured to inject
respective perturbation signals into the bypass current, where the
perturbation signals alter the first and second bypass currents in
a manner indicating impedance of the first and second batteries.
The controller may control the respective perturbation signals to
cancel out one another at a combined battery output. The first and
second bypass circuits may be communicatively coupled to the
controller via a controller area network (CAN) bus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The foregoing will be apparent from the following more
particular description of example embodiments, as illustrated in
the accompanying drawings in which like reference characters refer
to the same parts throughout the different views. The drawings are
not necessarily to scale, emphasis instead being placed upon
illustrating embodiments.
[0015] FIG. 1 is a diagram of a prior art battery including a
battery management system.
[0016] FIG. 2 is a diagram illustrating parallel battery
configurations in the prior art.
[0017] FIGS. 3A-B illustrate a circuit for controlling a battery
current in one embodiment.
[0018] FIG. 4 illustrates a multi-battery power source in one
embodiment.
[0019] FIG. 5 illustrates a device for controlling a battery
current in one embodiment.
[0020] FIG. 6 illustrates a battery in one embodiment.
[0021] FIG. 7 is a flow diagram of a control process in one
embodiment.
[0022] FIG. 8 is a flow diagram of a control process in a further
embodiment.
[0023] FIG. 9 is a diagram of a battery model.
[0024] FIG. 10 is a timing diagram of a perturbation signal in one
embodiment.
DETAILED DESCRIPTION
[0025] A description of example embodiments follows.
[0026] FIG. 1 illustrates a typical prior-art battery 100 including
a battery management system (BMS) 150. The battery 100 includes one
or a plurality of battery cells 105 connected in series to an
output terminal pair, as well as one or a plurality of cell
balancing circuits 108 each connected in parallel to a respective
one of the battery cells 105. A load switch 106 is connected
between the batteries 105 and the output terminal, and provides
binary (on/off) control of the charge/discharge current of the
battery 100. The BMS 150 enables and disables the load switch 106
to control charging and discharging of the battery 100. To prevent
unsafe operation (e.g., excessive charge or discharge current), the
BMS 150 turns off the load switch 106 in response to detecting a
battery current above a safe threshold. The BMS 150 also controls
the cell balancing circuits 108 to perform cell balancing, thereby
maintaining the battery cells 105 at a common voltage or state of
charge.
[0027] A typical battery, such as a battery having a 6T form
factor, can be configured comparably to the battery 100 of FIG. 1.
In many applications, it is advantageous to replace older or aging
batteries (particularly batteries implementing inferior cell
chemistries) with newer batteries or batteries implementing
superior cell chemistries. For example, 24V Lithium-ion 6T
batteries are candidates as the drop-in replacement for existing
12V 6T absorbent glass mat (AGM) batteries due to their benefits,
including long lifetime, high energy density, light weight and low
cost of ownership. However, various chemistries are used for the 6T
Lithium-ion batteries depending on the considerations such as
safety, performance, stability, and cost.
TABLE-US-00001 TABLE 1 Typical properties of Lithium-ion battery
chemistry. Battery Nominal Energy Cycle Chemistry Voltage(V)
Density (Wh/kg) Life (cycles) NCA 3.6 100~150 2000~3000 LFP 3.3
90~115 >3000 LCO 3.7 100~190 500~1000 NMC 3.6 100~170 2000~3000
LTO 2.2 60~75 >5000 LMO 3.8 100~120 1000
[0028] As shown in Table 1 above, the Lithium-ion chemistries have
different properties, and the batteries demonstrate different
characteristics on voltage, ampere hour (Ah) rating, C-rate
performance, life cycles, ageing behaviors, etc. When batteries
with different properties (e.g., chemistries) are coupled to a
common load, the charging/discharging stress may not be distributed
evenly, and optimal battery performance may not be achieved.
Circulating current flowing among batteries at different SOCs or
voltages can occur, and can result in over-charging,
over-discharging, early failure of the batteries. Further, for
batteries with the same chemistry, due to parameters tolerance,
similar performance degradation also applies.
[0029] Therefore, a solution is needed to ensure a power source
operates optimally when component batteries with the same chemistry
and different chemistries, such as different Lithium-ion 6T
batteries, are paralleled.
[0030] FIG. 2 illustrates prior art circuits 201, 202, 203 having
parallel battery configurations. Each of the circuits 201, 202, 203
includes plural batteries 205 connected in parallel between an
energy source 204 and a load 207. Each of the circuits 201, 202,
203 also includes a circuit element in series with each of the
batteries 205 without a load switch: the circuit 201 includes
resistors 211, the circuit 202 includes diodes 212, and the circuit
203 includes DC-DC converters 213. The resistors 211 and the diodes
212 can contribute to limiting the battery imbalance and
circulating current. However, the power loss associated with the
resistors 211 and diodes 213 render the circuits 201, 202
impractical in many applications.
[0031] In the circuit 203, the DC-DC converters 213 can contribute
to providing a stable output voltage at the power bus despite
voltage or state of charge (SOC) differences among the batteries
205. However, the circuit 203 alone may be ineffective or
impractical in many applications. For example, in applications of a
6T format battery, a DC-DC converter meeting the peak power
requirement (1100 A) will be too large to fit into the 6T form
factor. The power loss resulting from the DC-DC converters would be
large when a high charging and discharging current passes through
the converter. Further, the DC-DC converter operation does not
consider individual battery cell performance and ageing, and
therefore cannot ensure the optimal operation of each battery.
[0032] For battery protection under conditions including
over-current, over-discharge, over-charge, a load switch is
typically used for on/off control of the current. However, the load
switch is not able to provide the optimum balancing current. The
more sophisticated method uses a DC-DC converter to regulate the
charging and/or discharging current. Ideally, the DC-DC converter
can provide optimum balancing current. But the issue in many
applications is that there could be substantially high pulsing
current load which requires over-design of the DC-DC converter.
This leads to high cost and impractical size DC-DC converter.
Therefore, a better solution is needed to provide the optimum
battery balancing current at low cost and small size and can
support high pulsing current load.
[0033] Example embodiments provide improved performance of
batteries during charging and discharging operation. When
implemented in multiple battery configurations, example embodiments
can also optimize charging and discharging of each battery based on
the properties of the battery as well as the properties of the
other batteries in the assembly.
[0034] FIGS. 3A-B illustrate a control circuit 310 for controlling
a battery 300 in one embodiment. The control circuit 310 may be
integrated with the battery 300 (e.g., included as a component of a
BMS (not shown)), or may be connected to the battery 305 as a
standalone device. The control circuit 310 includes a bypass
circuit 330 and a controller 320 and load switch 306 if load switch
is not available in existing battery setup. In example embodiments,
the controller 320 may include a buck-boost bi-directional power
train, where the bypass circuit 330 includes a bidirectional
converter such as a buck-boost converter, a boost-buck converter, a
buck converter, a boost converter, or a dual-active bridge (DAB)
converter. The bypass circuit 330 is connected to a terminal of a
battery 305 (directly or via one or more intermediary circuit
elements) and is connected in parallel with a load switch 306. By
selectively passing current between the battery 305 and an output
terminal, the bypass circuit 330 can selectively direct a bypass
current around the load switch. Alternatively, a plurality of
battery cells connected in series or parallel may be implemented in
place of the battery cell 305.
[0035] The controller 320 may be configured to operate in plural
modes to control the bypass circuit 330. In a first mode shown in
FIG. 3A, the load switch 306 is in an off state, and the controller
320 controls one or more parameters of the bypass current based on
a state of the battery 305 and/or a state of one or more other
batteries (not shown) that make up a common power source. For
example, the controller may receive, measure, or calculate values
indicating current at the battery terminal (e.g., charge current or
discharge current), a voltage and/or impedance of the battery,
and/or a temperature of the battery. Based on some or all of those
values, the controller 320 may control the bypass circuit to direct
a bypass current around the load switch 306. The bypass circuit 330
may control the bypass current by controlling parameters of the
current such as magnitude (of a DC current) or frequency/period (of
an oscillating or pulse-width modulated (PWM) signal). As a result,
the bypass circuit 330 can pass a controlled charge or discharge
current when the load switch 306 is off. For example, when a low
charge or discharge current is required (e.g., to reduce battery
temperature, or to balance operation with other batteries), the
bypass circuit 330 can pass a bypass current that is low relative
to the current that is passed by the load switch 306 in a on state.
The bypass current can also be varied dependent on its application
and/or the observed battery states described above. By providing a
controlled bypass current, the control circuit 310 can provide a
variable-current alternative to the binary states provided by the
load switch 306. Example applications of the bypass current, as
well as corresponding operation of a control circuit, are described
in further detail below.
[0036] The controller 320 may also control the bypass circuit 330
to convert a discharge current to an output current having a higher
voltage and a lower current. As a result, battery 300 can be
operable in some applications requiring a higher voltage power
source without the need for additional batteries.
[0037] In a second mode shown in FIG. 3B, the load switch 306 is in
an on state, and the controller 320 controls the bypass circuit 310
to disable the bypass current. The bypass circuit 330 may pass a
relatively small ("leakage") current in this mode, while the load
switch 306 passes a relatively high current for charging and/or
discharging the battery 305. During a charge or discharge
operation, the battery 300 may enter this mode when higher current
is permitted and observed battery conditions do not necessitate a
lower, controlled bypass current. The battery 300 may also enter an
off state (not shown) where both the load switch 306 is off and the
bypass current is disabled.
[0038] FIG. 4 illustrates a multi-battery power source 400 in a
further embodiment. The power source 400 may include two or more
batteries (two of which are shown) coupled in parallel to a common
power bus. Each of the batteries 405a-b may be configured
comparably to the battery 300 described above, including a
respective control circuit 410a-b having a controller 420a-b and a
bypass circuit 430a-b connected in parallel with a load switch
406a-b. In a first mode of operation wherein a respective load
switch 406a-b is off, one or more of the controllers 420a-b may
control respective bypass currents as described above.
[0039] Further, the controllers 420a-b may control the bypass
currents based on information about other batteries in the power
source 400 other than the battery to which it is connected. For
example, the controllers 420a-b may control the bypass current
based on the temperature of the connected battery (e.g., a detected
temperature or a value corresponding to the temperature), or a
difference between the corresponding temperature and a temperature
of at least one additional battery. Specifically, a hotter battery
may be controlled to exhibit a lower bypass current for charging or
discharging, and a cooler battery may be controlled to exhibit a
higher current via the bypass circuit 430a-b or the load switch
406a-b. Each controller 420a-b may also adjust the bypass current
over time in response to changing temperatures of the corresponding
battery and/or other batteries, for example when those battery
temperatures surpass or fall below predetermined thresholds. As a
result, the power source 400 can provide a desired current at the
power bus while preventing each of the batteries 405a-b from
overheating.
[0040] In a further example, the batteries 405a-b may differ in one
or more properties or states, such as capacity (e.g., 100 Ah versus
50 Ah) or output resistance as a result of different numbers of
past charge-discharge cycles or battery chemistries. In the case of
different capacities, a lower-capacity battery may have a lower
output resistance, meaning that it may discharge faster than a
higher-capacity battery under the same load. To address this
problem, a controller at the lower-capacity battery may control a
lower bypass current to discharge the battery at a slower rate. The
higher-capacity battery, in contrast, may be discharged at a full
current or via a higher bypass current. As a result, the batteries
may discharge at a comparable rate.
[0041] In the case of batteries having different measures of ageing
(e.g., charge/discharge cycles), a newer battery may have a lower
output resistance than an older battery. To detect and compensate
for this difference, the controllers 420a-b may measure the output
current of each battery 405a-b, measure and communicate impedance
of the batteries, and may retrieve reference information about the
batteries (e.g., model number, date of manufacture, number of
charge/discharge cycles). Based on some or all of the above
information, one or more of the controllers 420a-b may control a
bypass current to provide for discharging the batteries 405a-b at a
comparable rate.
[0042] The controllers 420a-b may also control the respective
bypass currents based on a received command signal indicating an
operational profile, where the operational profile can indicate
desired operational metrics for the entire power source 400 or an
individual battery, such as a desired discharge current, discharge
voltage, charging current, and/or a threshold battery temperature.
The controllers 420a-b may also control the respective bypass
currents based on a status signal, where the status signal
indicates temperature, current, capacity, impedance and/or voltage
of one or more batteries of the power source 400.
[0043] To communicate such operational profiles or status signals
among the controllers 420a-b, the control circuits 420a-b may be
communicatively coupled to one another via a data bus (e.g., a CAN
or J1939 bus). The control circuits 420a-b may also communicate by
injecting perturbation signals into the bypass current that can be
detected by another of the control circuits. For example, the
controller 420a may inject a perturbation signal that alters its
bypass current in a manner indicating impedance of the battery
405a. This perturbation signal can then be measured by the
controller 420b to determine the impedance of the battery 405a.
Example operations using perturbation signals are descried in
further detail below. The controller 420b may also calculate
impedance of the battery based on a change in voltage and/or
current at a power bus terminal.
[0044] In a further example, the control circuits 420a-b may
communicate with one another to exchange information about each of
the batteries 405a-b and, based on that information, determine
charge and/or discharge currents for each of the batteries 405a-b
to be enforced via the bypass circuits 430a-b. For example, the
control circuits 420a-b may communicate with one another to
exchange some or all of the following information: [0045] 1)
Manufacturer part number (e.g., SAFT, Bren-Tronics, A123,
Panasonics, etc). The controllers can fetch a database for the part
number and/or model either from a local database or from the
communication bus. [0046] 2) A data code from which battery ageing
related to time can be derived. [0047] 3) Usage history: The stress
the battery has endured during its usage. [0048] 4) A battery
charging and discharging real-time model: This model can be used to
calculate the stress during the charging and discharging based on
the battery or battery cell voltage, current and temperature. For
example, the battery cell temperature or the derived temperature
stress can be used in this model to show that, with existing
current parameters, the battery will lose 5% of its capacity in the
next 10 cycles. [0049] 5) Battery capacity, SOC, and
state-of-health: This is used to be compared with other batteries.
For example, if battery A has half of battery B's capacity, the
expected current of A is around half of that of battery B with
other factors being equal.
[0050] Items (2) and (3) may be entered into the model/database of
(1) to estimate the battery ageing. As a result, the ages of the
different battery can be compared accordingly. For example, based
on the data code and history usage, the controller determines that
the battery has a given number of cycles left within its
lifespan.
[0051] Therefore, using (1), (2) and (3), a battery's age may be
calculated. For example, batteries 405a and 405b are configured in
parallel, it is desired that both batteries reach the end of life
at the same time. Provided that battery 405a has 250 cycles left
and battery 405b has 500 cycles left, then the stress reference for
battery 405a should be twice of that of battery 405b.
[0052] Item (4) can be used to provide stress feedback. Based on
existing current, voltage and corresponding temperature, the ageing
rate of the battery can be calculated. The stress reference from
(1)(2)(3) and stress feedback (4) can be compared, and the error
will be fed into a control algorithm to control the current. The
control algorithm can factor in the items in (5), and the
controllers 420a-b can control the current accordingly. Under
certain load conditions or charging conditions (e.g., a demand for
maximum current), such control may not be permitted. In such a
case, the controller will control the current when permitted.
[0053] FIG. 5 illustrates a device 500 for controlling a battery
current in a further embodiment. The device 500 may be implemented
as an external unit for connection with an existing battery 505
(e.g., battery cell or multi-cell battery pack). The device 500 may
include some or all features of the control circuits 310, 410a-b
described above, including a controller 520 and a bypass circuit
530. The device 500 may also include a load switch 506 operated by
the controller 520 according to a determination on whether to
direct a "full" current, a controlled bypass current, or no
current. The battery 505 may include an integrated load switch (not
shown) that cannot be bypassed by the device. In such a case, the
device 500 may pass a bypass or full current when the integrated
switch is on. The device 500 may control the parameters of the
bypass current as in the embodiments described above. In
particular, the controller 520 may detect temperature of the
battery 505 via a sensor communicatively coupled to the controller,
detect current through the battery terminal via a current sensor,
detect voltage across the battery 505, calculate impedance of the
battery 505 based on the voltage and current, calculate impedance
of other batteries, and/or receive information about other
batteries via a data bus. Based on some or all of the above
information, the controller 520 may control the bypass current to
perform battery balancing, stress reduction, temperature reduction,
or other functions described herein.
[0054] FIG. 6 illustrates a battery 600 in one embodiment. The
battery 600 may be implemented as a battery pack such as a 6T
battery, and includes a plurality of battery cells 605 connected in
series to an output terminal pair, as well as a plurality of cell
balancing circuits 608 each connected in parallel to a respective
one of the battery cells 605. A load switch 606 is connected
between the batteries 605 and the output terminal, and provides
binary (on/off) control of the charge/discharge current of the
battery 100. A BMS 650 enables and disables the load switch 606 to
control charging and discharging of the battery 600.
[0055] A controller 620 and a bypass circuit 630 may include some
or all features of the control circuits 310, 410a-b described
above. However, the controller 620 (or functionality thereof) may
be incorporated into the BMS 650. Thus, in addition to performing
battery management functions such as controlling the load switch
606 and the cell balancing circuits 608, the BMS 650 (via the
controller 620) may also control the bypass circuit 630 to perform
one or more of the bypass operations as described above.
[0056] FIG. 7 is a flow diagram 700 illustrating an example process
carried out by a controller 720 to determine parameters of a bypass
current and control a bypass circuit 730 accordingly. One control
strategy is to reduce the temperature stress the battery encounters
during battery charging and charging by following a control
structure.
[0057] The battery temperature affects its lifespan and performance
significantly. For example, lithium-ion battery often achieves its
best performance and longest life time at a temperature of
.about.20 C. Too high or too low a battery temperature will result
in detrimental effects to its output energy, operating time and
life. Meanwhile, during charging and discharging, a battery will
generate heat. The heat generated is related to operating current
and state-of-charge (i.e., how full the battery is). In general,
the heat generated increases when charging/discharging current
increases. Further, the relationship between the current and the
generated heat is a non-linear process and more than proportional.
For example, a double of output current will result in
substantially more than twice the generated heat. Therefore, to
reduce heat and resulting thermal stress, the operation current can
be kept low and current proportionally distributed among
batteries.
[0058] Further, a battery generally has a substantial thermal mass.
That is, when heated, the battery will take a long time to warm up.
Although the exact time constant depends on battery's design,
usually it is at the order of minutes to tens of minutes.
Therefore, when heat is generated, it takes a substantial amount of
time (e.g. 30 min.) for the battery temperature to rise to its
equilibrium. Further, when a battery has been warmed up, it takes a
long time for the battery to cool down even the heat is removed.
During the cooling period, the battery still sees high temperature,
which continues degrading the battery. Moreover, a battery may
undergo a greater and faster temperature rise in response to a
greater charge or discharge current.
[0059] The flow diagram 700 illustrates a control process to reduce
the temperature stress the battery experiences during battery
charging and charging. At block 701, the controller 720 may
calculate the battery heat generation during charging, discharging
or continuous cycles of charging and discharging. Different
batteries may have different heat generation model, and the
specific model can be known from battery vendor or from analysis.
At block 702, the controller 720 may simulate the battery thermal
mass (emulating the slow temperature change) as a multiple-order
filter to reflect the thermal stress at the time domain. The block
702 output may be average temperature stress for a pre-defined
period of time or RMS value of the temperature stress for a
pre-defined period of time. Further, the calculated result can be
normalized with the stress value calculated at a pre-defined
operation condition (e.g. 1 C discharging and/or charging) so that
different batteries can be compared.
[0060] Through blocks 701 and 702, the controller 720 may calculate
the thermal stress on the battery. This stress may also be
determined from the temperature reading from the battery. However,
it may be more effective to set the battery thermal stress model
based on local battery information to represent the thermal effects
on battery because the temperature reading might not correlate to
battery thermal stress closely. At block 703, the controller 720
may receive stress information from other controllers and may
calculate how much stress the local battery should have as a share
of the total stress. Based on the calculations at blocks 701-703, a
power controller 722 may then control the bypass circuit 730 to
direct a bypass current having corresponding parameters.
[0061] In an example calculation at block 701, a signal correlated
to the heat may be calculated as:
Heat=(Vo,cal-Vout)*Iout+Iout 2*Rout+delatH
[0062] Where Vo,cal is the calculated battery open loop voltage,
Vout is the battery voltage under the load condition, Rout is the
battery internal resistive component, and deltaH is other
battery-related heat generation related to specific battery.
[0063] At block 702, a multi-order or a first-order filter
(1/(TcS+1)) may be used to simulate the thermal stress with first
order approximation. Not shown here is the average and RMS value
calculation for a pre-defined time. The calculated result can be
normalized. At block 703, the normalized values can be shared among
batteries. Further, historical information of the battery history,
the battery life form battery vendors (e.g. as curves, a chart
and/or table), and real time measurement such as electrochemical
impedance spectroscopy (EIS) or impedance, current can be used for
setting the reference stress, so the power controller can take
control actions.
[0064] The power controller 722 may regulate the local stress to
match the reference. This process may not be real-time control, and
may carry out the following control rules: (1) Try to reduce
thermal stress at the beginning of the operation because any
temperature rise at the early stage will affect late operation due
to the big thermal mass. (2) Load predictions will be used to
predict the load condition. (3) If used, try to keep the power
stage in buck mode to improve overall performance.
[0065] FIG. 8 is a flow diagram illustrating a process 800 of
determining a bypass current in a further embodiment. The process
800 may be implemented by a controller described above, and
utilizes a battery model to determine the bypass current, where a
life model 860 and a performance model 870 are components of the
battery model. The life model 860 represents the remaining life
time of a battery. As the battery ages, it may not be capable of
charging or discharging as specified like a new one. Further, its
internal power loss will grow during operation. Accordingly, the
life model 860 will include lossy elements as part of a "life time"
indicator, and it changes according to an ageing estimation. The
battery performance model 870 is used to simulate the internal
chemical characteristics of the battery. Therefore, this model can
predicate operating parameters such as voltage supplied to the load
when a certain type of load is connected. One of the important
performance parameters is state of charge (SOC), which indicates
how much charge, or energy can be stored in the battery.
[0066] At block 801, the controller may performs two jobs: 1)
continuously refine the life mode 860; and 2) provide a current
stress sensing result for determining the charging/discharging
control value. Here, the controller may accept the real-time
operating data, including SOC, temperature, discharging/charging
rate from the battery. Based on the previously established life
model, the controller can generate real-time stress information.
The life model parameter may be continuously updated along with
usage of battery.
[0067] At block 802, the controller tracks the battery SOC changes
with charging and discharging operations, thereby providing
real-time SOC information.
[0068] At block 803, the controller may apply a Kalman filter,
Butterworth filter, or any other filter to refine the performance
model based on SOC and EIS. Impedance (which varies with SOC) can
be estimated from the performance model 870 using SOC information.
If the estimated impedance is different from actual measured
impedance, the controller may update the model to reflect the
difference.
[0069] At block 804, the controller may perform a normalization
conversion. Individual batteries can exhibit substantial
differences in characteristics such as chemistry and volume,
resulting in different SOC and impedance. Normalization converts
the measured values to values suitable for comparison across
different batteries. For example, for a 60 Ah battery, 1 C rate is
60 A. For a 500 Ah battery, 1 C rate is 500 A. If we only compare
60 A vs. 500 A, it may indicate 500 A is high stress. However, if
we normalize the charging current by Ah, then both are equal to 1,
which shows that the stresses are the same. After normalization,
the stress factor can be used to compare with another battery's
same normalized value. Based on this comparison, the controller can
determine an appropriate bypass current and control a bypass
circuit accordingly.
[0070] As described above with reference to FIG. 4, control
circuits in example embodiments (e.g. control circuits 420a-b) may
communicate by injecting perturbation signals into the bypass
current that can be detected by another of the control circuits.
For example, the controller 420a may inject a perturbation signal
that alters its bypass current in a manner indicating impedance of
the battery 405a. This perturbation signal can then be measured by
the controller 420b to determine the impedance of the battery 405a.
Further, when 420a-b are measuring impedance locally, in order to
avoid distortion in the combined current of the system, the control
circuits may inject perturbation signals that, when combined,
cancel one another out.
[0071] FIG. 9 is a circuit diagram of a battery model 900 that may
be referenced to calculate an appropriate perturbation signal. The
battery can be simplified to model as an ideal DC voltage source
Vbatt in series with an impedance Z. Z is formed by series/parallel
connection of basic passive electrical elements such as resistors,
capacitors and inductors. An EIS method can be used to measure this
impedance Z. Because of the existence of capacitor and inductor, Z
is a function of frequency. Thus, an EIS process may scan the
impedance across the range of frequencies that of interests.
Typically, it is from 0.01 Hz to 10 kHz. The result will form Z
over frequency f as Z(f).
[0072] To achieve that, a current perturbation "i" is injected to
the battery. "i" is a sinusoidal signal at a frequency of test.
Alternatively, "i" may be a periodic signal that can be decomposed
into its sinusoidal Fourier terms. Further, the perturbation can be
from the load, and the perturbation injection is not needed.
Because Vbatt is an ideal DC source, from sinusoidal signal at a
frequency f above DC, it is virtually short. Thus the model at
testing frequency f (f is higher than 0 Hz, or DC) is comparable to
the model 900 with the omission/short of the battery.
[0073] Provided that the voltage across the battery is measured to
obtain Veis, and focusing on the Veis value at this testing
frequency, Z can be obtained as:
Z(f)=(V.sup..about._eis(f))/(i.sup..about.(f))
[0074] The ".about." above the name indicates that it only refers
to the signal content at the frequency of interest "f." An example
waveform is shown in FIG. 10. Here, frequency is "f", ".theta." is
the phase-shift between the voltage and current at frequency f.
[0075] While example embodiments have been particularly shown and
described, it will be understood by those skilled in the art that
various changes in form and details may be made therein without
departing from the scope of the embodiments encompassed by the
appended claims.
* * * * *